The present invention relates to an optically pumped amplifier, especially a solid-state amplifier, with an amplifying medium and with an optical pumping array, by means of which the pumping radiation is coupled to the amplifying medium; whereby the pumping radiation is formed before the coupling; and whereby the volume of the amplification medium is only partially pumped.
Optically pumped amplifiers in the form of laser arrays have made their entry into almost all areas of technology. Current developments in the area of laser arrays are directed, among other things, toward increasing the output, improving beam quality, and forming and transforming the output radiation in a defined way.
One class of lasers which, in recent years, has found increased applications in materials processing and medicine is solid state lasers. They are distinguished in that with them, particularly in low power classes, high beam quality can be produced. Such solid state lasers are further distinguished by small attainable design sizes, typically with a length of about 8 cm and a diameter of 1 cm.
While solid state lasers have been pumped in the past using lamps, increasingly the solid state medium is pumped by means of diodes or diode fields. As opposed to lamp-pumped solid state lasers, diode-pumped solid state lasers are distinguished, among other things, by high efficiency, high beam quality, long service life and small dimensions. These can be attained especially with a diode pump array. Various types of laser systems can be implemented in connection with diode-pumped laser arrays. In solid state lasers, distinctions are principally made between axially and transversally pumped solid state lasers (for example, solid state lasers pumped using diodes). Generally the axial pumping array for lasers is used with an output up to several tens of watts, while the transverse pumping array is preferred for scaling the output up to several kilowatts.
The limiting factor for beam quality and output performance of optically pumped amplifiers, which also holds true for diode-pumped solid state lasers, is thermal interference. This is caused by unavoidable heat losses in the amplification medium and in the lasing medium. Additionally, the adjustment between the pumped volume and the mode volume of the resonator plays a decisive role in efficiency and beam quality. To comply with these requirements, the amplification mediumxe2x80x94in solid state lasers, it is the solid state mediumxe2x80x94is pumped via the end (xe2x80x9cend-onxe2x80x9d), making possible an optimal overlap of active volume and mode volume. If diode lasers or diode laser arrays or field arrays are used for such pumping, the radiation of diode lasers, being asymmetric by nature, is so formed that it can be focused on a circular spot. The homogenized radiation is then coupled through the end into the solid state medium, as is also depicted in FIG. 19 from the drawings. If the resonator is designed so that the mode diameter roughly corresponds to the pumping spot diameter, then the laser can be operated efficiently with a high beam quality.
One problem that exists with end-on pumping is that relatively expensive beam formation is needed to focus the pump radiation, and that the limited pump volume limits the attainable laser performance.
To scale the laser output to higher performances, laterally and transversely pumped arrays are used. One such array is depicted in FIG. 20. One such coupling of pump radiation is designated as closed coupling in the technical terminology. It is distinguished by its simple design. It is true that attainable laser performance per length, particularly for pulsed lasers, is limited, since only limited pumping performance can be made available with this array. For this array, high fabrication precision in relation to the relative position of diode laser billets to the rods to be pumped is required. Otherwise, a large part of the diode radiation cannot be coupled into the amplification medium owing to the large divergence angle.
Additional arrays for pumping of solid state bodies according to the state of the art are depicted in FIGS. 20 to 22. In accordance with these arrays, the highly divergent diode laser beams are coupled using cylindrical lenses or elliptical cylinder mirrors into the solid state medium. In these arrays, the gain and amplification distribution can be optimized, depending on the application, through varied focusing. However, focusing components are required, which considerably increase the fabrication costs of the arrays.
The previously mentioned pumping arrays are used for amplification media, i.e. as regards solid state lasers, the solid state media, in the form of rod geometries. It is true that similar pumping arrays can also be used for amplification media or solid state media with so-called slab geometries or plate geometries. Two examples of arrays which preferably can be used in connection with plate-shaped amplification media, are depicted schematically in FIGS. 23 and 24 of the drawings. In the pumping array depicted in FIG. 23, the radiation of the diode laser stack is coupled to the amplification medium by means of a so-called non-imaging concentrator. It is in fact difficult with such an arrangement to illuminate the amplification medium uniformly, i.e., homogeneously, from all sides. Optimization regarding this is attained with the array of FIG. 24, with the diode laser beam coupled through the two narrow sides into the plate-shaped amplification or solid-state medium. Here also it is in fact difficult to achieve a homogeneous optical stimulation or irradiation of the plate-shaped solid-state medium, and thus a homogeneous pumping distribution within the solid state medium.
One feature common to the previously described arrays is that the amplification medium (solid state medium) is pumped in full-volume fashion by means of the pump radiation (diode laser radiation). Owing to this, an amplification profile is produced that is clearly defined by the dimensioning of the amplification medium in all directions. Fundamentally, however, it is not possible that such a clearly delimited amplification profile is fully covered through the laser mode. This, however, is a prerequisite for efficient laser operation and high beam quality of effective radiation. Additionally, the measurements of a solid state medium that is pumped with diode laser radiation cannot be kept arbitrarily small. This is because of the particular absorption coefficient of diode laser radiation. To operate the laser with a high beam quality, therefore, the laser mode volume must be selected to be appropriately large. This, in turn, results in a resonator length which is technically difficult to master. Not lastly, the attainable output performance is limited by thermal disturbances, such as birefringence and thermal lenses.
Proceeding from the state of the art described above, and the problems connected therewith, the task of the present invention is to provide an optical amplifier in which it is possible to attain an optimal overlap of the pumped volume through the mode volume while simultaneously minimizing the thermal degradation, thermal aberration and depolarization loss.
In regard to optically pumped amplifiers, especially in regard to solid state amplifiers of the types previously described, this problem is solved by having the volume of the amplification medium only partially pumped. The pumped volume of the amplification medium in cross section exhibits an approximately rectangular cross section perpendicular to the optical axis. By means of these measures, through suitable coupling arrangements, efforts are made to have only defined partial volumes of the amplification medium (such as a solid state medium) by means of pumping radiation, preferably in connection with solid state amplifiers by means of diode laser beams, with an approximately rectangular cross section. A defined rectangular cross section of the amplification medium can be pumped, and in fact independently of its actual cross sectional form. The latter could also be of circular shape, for example. What is attained by this is that an optimal overlap in regard to the beam quality efficiency is possible, particularly in connection with an off-axis, unstable resonator. There is a quasi-one-dimensional heat transfer, and thus, minimal depolarization loss.
By the term xe2x80x9camplification mediumxe2x80x9d, the description, what is to be understood is a medium that contains excitable atoms, molecules, ions or excimers, by means of pumping radiation. The term amplification medium is also used in the description when only partial pumping or excitation takes place.
Because of the small dimension in relation to the amount of the pumped volume, a small thermal lens effect is achieved by the specific measures according to the invention.
Additionally, only extremely low depolarization losses occur, since in this case there is a quasi-one-dimensional heat transfer. By means of the defined, rectangular volume excitation with pumping radiation, an effect on the beam quality can be attained. This is done by having the height of the pumped volume cross section designed so that it approaches the dimension of the ground mode (ground mode diameter), resulting in a higher attainable efficiency. These advantages are to be particularly cited in relation to solid bodies which are used as amplification media. Additionally, they are to be cited precisely when such solid state media are pumped with diode radiation, for it is precisely with solid state media that the invention-specific measures can implement such pumping geometries relatively simply and efficiently.
What is preferred is an adjustment of the ratio of the maximum to the minimum cross sectional width of the amplification medium pumped volume, viewed as perpendicular to the optical axis of the amplification medium. This adjustment is done so that it amounts to less than 1:5. This means that the fluctuation width of the optically pumped zone in the amplification medium, by such means as by one or more constrictions, is kept within defined small limits.
Additionally, the relationship of width to height of the rectangular cross section of the pumped volume must be greater than 1.8, so that an elongated cross sectional volume is pumped in the amplification medium. In contrast to a rectangular-cross-section pumped volume, by this means an advantage is achieved in that a quasi-one-dimensional heat transfer is present. Connected with that is minimal depolarization loss.
Approximation of the pumped volume to a rectangular cross section can be simplified by having the amplification medium pumped from two opposite sides, and approximately perpendicular to the optical axis. This is called transverse pumping. A further optimization to excite an approximately rectangular cross sectional volume by means of optical pumping radiation can be attained if the amplification medium is pumped from two opposite sides that are roughly parallel to the optical axis. This is called axial pumping. By this means, an approximately ideal rectangularly pumped volume can be achieved.
As was already mentioned previously, the invention-specific measures in particular offer advantages in connection with amplifiers in which the amplification medium is a solid state medium. In connection with such solid state media, this can be divided into various zones which are subjected to different doping. These zones can be formed either along the optical axis, or in fact perpendicular to it. With varying doping in the direction of the optical axis, the pumping performance density can be controlled with axial pumping. With doping that is altered in a direction perpendicular to the optical axis, the gain profile can be adjusted to the requirements. Preferably, the doping decreases from zone to zone toward the pumping source. By this means, homogeneous pumping along the pumping direction can be attained.
In connection with a solid-state medium as an amplification medium, optical pumping is done preferably by layers. For this purpose, the amplification medium is divided into fictitious, layered sections, preferably parallel to the optical axis, which are then pumped with varied pumping radiation. By this means, the pumping performance is increased, and thus also the laser performance.
If the solid state medium, for example in relation to a layering, as is presently indicated, is divided, it can be useful to insert a cooling device between each two solid state media, in order to remove heat, thus further increasing attainable performance per length.
As a pumping source of the pumping array by which the amplification medium is optically pumped, it is preferable to use diode lasers or diode laser arrays. These diodes or diode laser arrays can be designed to be compact and stacked in a great variety of configurations. Thus the volume of the amplification medium can be pumped in defined fashion, particularly in reference to the previously indicated, preferred embodiment forms, in which the volume of the amplification medium is divided into zones. Diode lasers and diode laser billets exhibit an elliptical beam cross section that greatly expands or diverges. For this reason, preferably the pumping radiation of a diode laser billet which is used for pumping, is collected by means of a cylindrical lens in linear fashion and/or focused, and coupled in defined fashion to the amplification medium with a narrowly limited pumping radiation cross section. The goal is to have pumping radiation which is high-power and can be emitted by a multiplicity of diode lasers, within a very narrowly defined volume of the amplification medium. To achieve this, several diode laser billets, which emit a quasi-linear-shaped output field, are combined into a field array. The output field of each individual diode laser billet is combined via a cylindrical lens assigned to this billet. The individual collimated radiation fields are then brought to an additional, focused cylindrical lens, from which the entire output field is coupled to the amplification medium.
As an alternative, initially the pumping radiation can be coupled into one or more optical waveguides by a suitable optical array. The pumping radiation emitted from the optical waveguides can then be coupled through an additional optical array into a section of the amplification medium.
To do optical pumping using diode laser billets of extended amplification media, several diode laser billets are placed next to each other, in the direction of the optical axis. Such a division has the advantage of permitting the laser power to be scaled practically at will.
Diode lasers of other beam sources, such as solid state lasers, excimer lasers, and/or ion lasers, can be used for optical pumping as pumping sources for the pumping array.
To fabricate a laser with the invention-specific amplifier, the amplification medium is placed within a resonator. Particular advantages can be achieved in relation to such a laser array if the resonator is designed so that in the width of the pumped volume an off-axis, an unstable resonator is formed, and in the height of the pumped volume a stable resonator is formed. It is exactly in connection with this resonator design that there are advantages in the invention-specific measures. High beam quality (also diffraction-limited beam quality) can be attained at high efficiency.
For stable resonators, the beam quality over the cross section(s) of the radiation emitted from the resonator can exhibit a certain inhomogeneity in both directions. Therefore, the radiation emitted from the resonator can be homogenized by an optical array. Such an optical array can be designed using such concepts as two step-like mirrors.
If necessary, the radiation emitted from the amplification medium can be converted by means of an etalon-shaped, non-linear medium. In connection with a laser array, the etalon-shaped medium or component can be placed inside or outside the resonator.
Yet another embodiment form, in connection with which the invention-specific pumping array can be used, is that in which the solid state medium is in the form of an optical waveguide. In one advantageous embodiment, this guide can have a doped core, preferably with a rectangular cross section. In this arrangement, the doping can differ between core and cover. It is precisely because of this that an extremely compact and disturbance-free laser call be produced.
Preferably in such an array, the pumping radiation is coupled to at least a front-side end of the cover and the core, and be brought within the cover. Typically such an optical waveguide can be 1 m long and have a diameter in the range from 5 xcexcm up to about a millimeter. As was indicated previously, with such an optical waveguide, a laser resonator can be fabricated, by having the resonator mirror placed on the two front surfaces of the optical waveguide. Such a laser is distinguished in that the large surface ensures effective removal of energy-loss heat.
Additionally, an optical waveguide has the advantage that it is via the large surface of an appropriately long optical waveguide that the energy-loss heat can be directed outward via the cover surface. For this purpose, two options are available. One is to mount the optical waveguide on a cooling plate and be in thermal contact with the cooling plate. The other possibility is to place the optical waveguide in a cooling chamber. Such a cooling chamber can be formed by having a hose around the optical waveguide, so that free space remains between the optical waveguide and hose. Through this space, a circulating fluid such as a coolant can be made to flow. The cooling cover and/or the coolant can assume a waveguide function for the pumping radiation.
There are instances when a long pump length must be attained in the pumping beam direction, particularly for the case of axial pumping. In such instances, the amplification medium should be pumped with radiation whose wavelength corresponds at least to a part of the weak absorption lines of the medium. In connection with a solid state medium doped with neodymium, it is pumped with pumping radiation whose wavelength is about 870 nm. This combination results in a highly efficient, long pumping extent in the direction of the optical resonator. By this means, possible parasitic oscillations can be suppressed.
Additional particulars and features of the invention can be gleaned from the following description of specific embodiment examples, using the drawings.